This invention relates to apparatus for providing high power amplification, usually at relatively high or microwave frequencies, through the use of a plurality of generally identical parallel connected amplifier elements. The invention particularly relates to such apparatus which provides a high degree of burn-out protection of the amplifier elements if the apparatus should become electrically unbalanced.
A number of different techniques for providing high power amplifiers for relatively high frequency signals are known which overcome the power limitations imposed by individual solid state amplifier elements. These techniques have commonly provided for parallel connection of a plurality of solid state amplifier elements, with each amplifier element sharing a portion of the amplification task, close to the limits of its capability. The total power output obtainable with such an amplifier is thus theoretically equal to the product of the number of amplifier elements used and the power output of a single element. An example of such a power amplifier is described in U.S. Pat. No. 3,593,174. Briefly, that patent describes a solid state apparatus for amplifying signals in the microwave frequency range and which comprises a plurality of parallel connected, solid state amplifier elements, connected at equally spaced distances between tapered input and output transmission lines which, respectively, distribute microwave power equally to each of the parallel connected amplifier elements and provide for phase coherent addition of the signal outputs from each of the amplifier elements. Using this technique of phase coherent addition of the signal outputs of the plurality of the amplifier elements allows each amplifier element, under normal conditions, i.e., when the device is in electrical balance, to assume its proportionate share of the amplification task. However, under unbalanced electrical conditions such as might be caused by an unfavorable voltage standing wave ratio (VSWR) condition on the output side of the amplifier or, as another example, upon initial calibration or trimming of the amplifier before it is brought into electrical balance, the amplification task is distributed unequally among the various amplifier elements with the result that certain elements can become overloaded. One method of protecting the individual elements in the case of an electrically unbalanced condition is to provide more amplifying elements than might otherwise be required so that each is operating well below it rating during normal operation. In case of electrical unbalance no amplifying element would then be subjected to potentially catastrophic overload. Another means of protecting the amplifier elements comprises a circulator connected between the amplifier output terminal and the load to exclude load reflected power from the amplifier.